Advances in light-emitting diodes (LEDs) have made light sources constructed from such devices attractive alternatives to conventional light sources such as fluorescent lights and incandescent lights. LED-based light sources have energy conversion efficiencies approaching or exceeding those of these conventional light sources. In addition, the LED-based light sources have lifetimes that far exceed those of these conventional light sources. For example, a fluorescent light source has a lifetime of about 10,000 hours whereas an LED has a lifetime of 100,000 hours. In addition, fluorescent light sources tend to fail completely without warning. In contrast, LED-based light sources tend to fade over time or shift in color, and hence, the user has adequate warning so that the light source can be replaced before it completely fails.
Fluorescent light sources are typically constructed from a glass tube containing a gas such as mercury vapor. The gas emits light in the UV range when the gas is excited by an alternating electric field created by applying a relatively high voltage between the ends of the tube. The UV light is converted to light of the desired spectrum by a layer of phosphor on the walls of the tube. Hence, the output light spectrum of fluorescent lights is limited to spectrums that can be produced by phosphors that can be excited with relatively high efficiency. In contrast, LED-based light sources can be constructed to generate a wide range of colors by utilizing a combination of LEDs of different colors.
Both the mercury vapor and fluorescent materials present environmental problems when inoperative fluorescent tubes must be replaced. In addition, these materials present personnel hazards when a tube is accidentally broken during replacement. Further, a fluorescent light requires a transformer to convert the conventional line voltages to the high voltages required to drive the fluorescent light. These transformers represent a significant cost and present fire hazards.
The amount of light generated per unit length of the tube in a fluorescent light fixture is approximately constant. Hence, fluorescent lights are typically either a long straight tube or a tube that is bent to provide an increased length in a smaller overall area. For example, tubes that are bent to form a ring are common. Furthermore, fluorescent light fixtures with adjustable intensities are difficult to construct, and hence, the user can only control the level of light in a room by turning on or off specific light fixtures.
Given the large installed base of fluorescent light fixtures and the advantages that are inherent in LEDs and other semiconductor light sources such as lasers, systems that are designed to replace existing fluorescent lights with semiconductor-based light are very attractive. For example, light sources that are constructed from a plurality of LEDs embedded in a plastic tube are known to the art. The LEDs are spaced along the length of the tube to provide an approximation to a linear light source that can be used to replace a fluorescent tube of the same geometry. Unfortunately, LEDs are inherently point light sources, and hence, these light sources appear as a string of bright “dots” rather than a uniformly illuminated tube. While the bright dots can be muted somewhat by including scattering material within or on the surface of the tube, the result is still a poor substitute for a fluorescent tube. In addition, LEDs emit light in a relatively narrow cone of angles, and hence, these light sources have intensities that vary with the viewing angle.